Clinical and experimental evidence suggests that two major central systems play important roles in pain sensory mechanisms: 1) direct spinothalamic projections to posterior lateral (PO) and medial-intralaminar (M-IL) thalamus and 2) reticulothalamic projections from nucleus gigantocellularis (NGC) of medulla. Accordingly, the PO and M-IL thalamus and NGC will be sampled, in the awake, behaving squirrel monkey, for neurons responding exclusively or differentially to noxious heat. Antidromic and orthodromic central nervous sytem (CNS) electrical stimuli will be used to obtain hodological information. Monkeys working for food reward will terminate thermal stimuli delivered to either paw; the escape response will interrupt access to food. The latency and frequency of escape will be monitored for noxious and innocuous thermal stimuli. A computer will be used to control behavioral contingencies and correlate extracellular unit responses with behavioral events. Since there is experimental evidence that an apparently selective analgesia can be produced by electrical stimulation of selected CNS sites, such stiumuli will be used to further test neural-behavioral correlations during behaviorally identified analgesia.